I just bought a used 1984 Ford Econoline with a veggie conversion. When I flip to veggie, it runs well for a few minutes, loses power, and stalls. If I flip back to diesel for just a couple seconds, then flip back, it regains power, runs well for 4-5 more minutes, loses power, and starts to stall, unless I flip back. I only need to flip back for a couple seconds, when I feel power return, and run back on veggie again for a few minutes. Bad oil?